Have you seen people leaving twenty feet between their car and the one in front of them at a traffic light?

Yes, but very rarely.

Yes.  It's what you should do.  It gives you room to maneuver should the car in front of you not be able to move for some reason.

Yes I have. It's really annoying too, when I'm trying to get into a left turn lane and there's no room due to their excessive spacing.

Yessss they do my head in!

Better that than being rear ended, or rear ending the one ahead.